It is generally accepted that bags 4ft and under are designed for punching. Bags 4ft to 6ft are more suitable for punch/knee, elbow and kick techniques.

Kicks and knees can be several times more powerful than punches, resulting in high movement in the bag. Generally speaking, the longer than bag, the greater the weight and hence less movement resulting from being hit by the legs.

A general guide to boxing bags deadweight:
3ft bags are approximately 15-20kg's, used for boxing fitness. 3ft bags have the highest swing radius
4ft bags are approximately 18-25kg's, and can be used for professional boxing, and are generally okay for kick boxing if the bag is on the heavier side.  These bags have a moderate swing radius.
5ft bags are approximately 28-50kg's, and are suitable for boxing, kickboxing, muay thai and MMA. They have a limited swing radius.
6ft bags are approximately 28-60+kg's, and are suitable for professional boxing, kickboxing, muay thai and MMA. These bags have a very limited swing radius.

Swing radius can vary greatly depending on if the user strikes the bag as the bag comes towards them or if the user strikes the bag as the bag swings away. The correct timing is to meet the bag with strikes are the bags swings towards you.

The weight of boxing bags can very significantly based on the circumference of the bag, and the type of filling used. (circumference can vary from 30-45cm).
Punch Equipment offers Urban and AAA range that requires pallets to be delivered, along with the heavier Mil Spec range.

HOW TO CHOOSE:
Tier One bags - Bags made from synthetic sheet of PU casing materials with webbing strapping rather than chains. No foam lining.
The PU synthetic sheet casing is good for home use 1-2 times per week. Punch does not offer a synthetic casing PU boxing bag, these can only be found in online chain sport stores or toy stores. The major issue with this type of bag is that if it tears, the tear will progress rapidly. 

Tier Two bags - Bags made from Ripstop tarpoline casings with webbed straps. Ripstop is manufactured sandwich technology of heavy synthetic sheet welded over a middle layer of mesh fibers, similar to fiber glassing. Punch Equipment has a full range of this in the URBAN range. The AUS BOXER range also features this heavily. The major benefit of this tier is that if the bag tears, the tear will not continue to progress pass the initial puncture point.

Tier Three bags - Bags made from heavy duty Ripstop tarpoline casings with a chain hanging system. Punch Equipment's AAA range bags feature this quality. Some high end Aus boxer range bags also feature this. The major benefit to this tier is that the tear will no continue past the point of puncturing.
Tier Three A bags - Bags made from commercial grade Micro fiber casings with a chain hanging system. The AAA PUNCHTEX tear drop bag are the gold standard for this tier.

Tier Four bags - These bags are the gold standard in the industry, made from commercial grade ripstop with a density rating over 800+ with a heavy duty chain hanging system. The Punch Equipment Mil Spec range  of AAA bags is exclusively this tier.

These ratings include a commercial grade tie down feature to limit the bag swing in professional gyms.

By Bruce Townhill, Founder, Managing Director and Designer of all Punch gear